The wide spread popularity of microwave ovens for home use has initiated interest in food trays which can be used in either microwave ovens or convection ovens. Such food trays must be able to withstand oven temperatures which approach 200.degree. C. Such trays are of particular value as containers for frozen prepared foods. It is accordingly necessary for such trays to have good impact strength and dimensional stability at both freezer and oven temperatures. It is, of course, also important for such trays to be capable of withstanding rapid heating from freezer temperatures of about -20.degree. C. to oven temperatures of about 175.degree. C or even higher.
Containers which are capable of being heated in either convection ovens or microwave ovens are sometimes described as being dual-ovenable. Polyesters are highly suitable for use in making such dual-ovenable containers. However, it is important for the polyester to be in the crystalline state rather than the amorphous state in order to achieve satisfactory high temperature stability. Normally, polyesters will undergo crystallization by heat treatment at elevated temperatures and the crystallites formed will remain substantially stable up to near the melting point of the polyester. As a general rule, dual-ovenable containers which are comprised of polyester will be heat treated to attain a crystallinity of higher than about 25%.
Injection molding and thermoforming are widely known methods for forming thermoplastic polyester articles. In injection molding, the polyester is heated above its melting point and injected under sufficient pressure to force the molten polyester to fill the mold cavity. The molten polyester is cooled in the mold until it is rigid enough to be removed. The injection molding of a polyester composition containing 0.5% to 10% by weight isotactic polybutene-1 is described in U.S. Pat. No. 3,839,499. However, the injection molding method is generally not satisfactory for the production of thin walled articles, such as dual-ovenable trays, due to flow lines and layering which develop during the filling of the mold which lead to non-uniform properties, surface irregularities, and warping of the finished article.
Thermoforming is another process which is used commercially in the production of polyester articles. It is a particularly valuable technique for use in producing thin walled articles, such as dual-ovenable food trays, on a commercial basis. In thermoforming, a sheet of preformed polyester is preheated to a temperature sufficient to allow the deformation of the sheet. The sheet is then made to conform to the contours of a mold by such means as vacuum assist, air pressure assist, or matched mold assist. The thermoformed article produced is normally heat treated in the mold in order to attain a crystallinity of at least about 25%.
Crystallization rates can generally be improved by including a small amount of a nucleating agent in polyester compositions. For example, U.S. Pat. No. 3,960,807 discloses a process for thermoforming articles from a polyester composition which is comprised of (1) a crystallizable polyester, (2) a crack stopping agent, preferably a polyolefin, and (3) a nucleating agent. Polyester articles which are made utilizing such compositions generally have improved mold release characteristics and improved impact strength. Additionally, the utilization of such modified polyester compositions results in faster thermoforming cycle times due to the faster rate of crystallization which is attained.
U.S. Pat. No. 4,572,852 discloses a polyester molding composition which consists of (1) polyethylene terephthalate, (2) a polyolefin containing from 2 to 6 carbon atoms, and (3) an effective amount of a heat stabilizer. Thin walled thermoformed articles which are prepared utilizing such compositions exhibit improved impact strength and high temperature stability. For this reason dual-ovenable trays which are comprised of polyester/polyolefin blends are widely utilized commercially. Polyethylene terephthalate having an intrinsic viscosity of at least about 0.65 is widely utilized in such applications. It is necessary for the polyethylene terephthalate used in dual-ovenable trays to have an intrinsic viscosity of at least about 0.65 dl/g in order for the article to have sufficient impact strength at low temperatures, such as those experienced in a freezer.